1. Field of the Invention
The present invention relates generally to semiconductor imaging devices. Specifically, the present invention relates to a method and apparatus which enables multiple nondestructive read out of signals generated by an image sensor wherein fixed pattern noise is canceled out.
2. Description of the Prior Art
Active pixel image sensing (APS) systems typically include an array of active pixel unit cells each of which includes a semiconductor image sensing device and active electronic components. Charge coupled devices (CCD's), photodiodes, and charge injection devices are examples of semiconductor image sensing devices used in APS systems. APS systems also include electronic circuits for reading image signals generated by the pixel unit cells.
Problems associated with semiconductor image sensing devices include charge transfer efficiency and destructive signal readout. Recent developments in APS technology attempting to address these problems include active pixel image sensing circuit devices fabricated using semiconductor processes which maximize charge transfer efficiency and which are compatible with complementary metal oxide semiconductor (CMOS) technology. CMOS compatible APS technology is suitable for applications including video phones, home surveillance devices, robotics and machine vision, guidance, navigation, and computer inputs.
Fossum et al. (U.S. Pat. No. 5,471,515) describes a CMOS active pixel image sensing device. For purposes of disclosure, Fossum et al. is incorporated herein by reference and describes devices in which each pixel unit cell of an array includes an image sensing device and three transistors for readout, selection, and reset. A column parallel readout architecture is employed to read out images one row at a time from the array. Circuitry is provided in each column for correlated double sampling (CDS) and fixed pattern noise (FPN) suppression.
Each pixel unit cell includes an imaging structure, a reset transistor, an in-pixel source follower, and a row selection transistor. The imaging structure includes a photogate with a floating diffusion output separated by a transfer gate. A readout circuit, which is provided for each column of pixels, includes a load transistor of a first source follower and two sample and hold circuits for storing a signal level and a reset level. Each sample and hold circuit includes a switch, a capacitor for storing charge representative of the sampled level, a column source follower, and a column selection transistor to buffer the capacitor voltages. Correlated double sampling (CDS) is achieved by sampling both a reset reference level and a signal level. The difference between the signal level and the reset reference level represents the net signal induced by illumination of the imaging structure.
During a signal integration period, photo-generated electrons are collected under the photo-gate of the imaging structure. After signal integration, an entire row of pixels is read out simultaneously using a plurality of readout circuits. The output of each pixel is sampled onto a reset capacitor in the readout circuit at the bottom of the column by enabling the sample and hold switch corresponding to the reset capacitor. The photogate of the pixel unit cell is pulsed low to transfer the signal charge to a floating diffusion node. The new output voltage of the pixel unit cell is then sampled onto the signal level capacitor at the bottom of the column by enabling the sample and hold switch corresponding to the signal level capacitor. The stored reset and signal levels are sequentially scanned out through the column source followers by enabling column address switches.
Fossum et al. further describes a method and apparatus for suppressing fixed pattern noise (FPN) in the above described circuit. FPN may limit performance of an image sensor and is attributed mainly to threshold voltage variations between adjacent source-follower transistors in the readout circuits. A crowbar switch and two column select switches on either side of the crowbar switch are used to selectively provide a short circuit between the two sample and hold capacitors. These switches enable delta difference sampling (DDS) wherein the reset and signal levels stored in each column are read differentially as described above. Subsequently, the crow bar switch is pulsed to short the two sample and hold capacitors in the column that is being addressed. The outputs of the reset and signal branches are then again read out differentially. Accordingly, a voltage and signal are generated which are proportional to the threshold voltage difference between the source follower transistors. This reference level is then subtracted from the previous reading, and the offset due to threshold voltage variations is removed.
As mentioned, in the readout circuit described by Fossum et al, the crowbar switch selectively provides a short circuit between the two sample and hold capacitors after measuring the difference between the charge levels stored in each capacitor. The shorting of these two capacitors causes charge sharing wherein charge flows from the capacitor having a higher potential to the capacitor having a lower potential. Therefore, the original stored charge level in each capacitor is changed by activating the crowbar switch. As a result, it is not feasible to obtain more than one measurement of the charge levels stored in the capacitors. This presents a problem because a single measurement may be inaccurate. It would be more advantageous to be able to make multiple measurements of the charge levels stored in the capacitors. Multiple measurements would allow for statistical analysis, including averaging and interpolation, of the multiple measurements to arrive at more accurate determinations of image information. Multiple measurements would also provide advantages for purposes of motion detection and predictive coding processing for video applications.